Method of making resistor

ABSTRACT

A method of manufacturing a precision resistor element of a predetermined resistance value, the resistor element including a quantity of resistive material extending between two terminals comprising removing a first portion of the resistive material in a first area between the terminals for substantially increasing the resistance to obtain a first resistance value for the resistor element, and removing a second portion of the resistive material in a second area alongside of the first area and between the first area and one of the terminals for slightly increasing the resistance to obtain the predetermined resistance value.

United States Patent [191 Cocca et al.

[ 1*Jan. 29, 1974 METHOD OF MAKING RESISTOR [73] Assignee: Spacetac Incorporated, Bedford,

Mass.

[58] Field of Search... 117/217, 212; 338/308, 262, 338/195; 29/620; 317/261 [56] References Cited UNITED STATES PATENTS 3,284,878 11/1966 Best 29/620 2,597,674 5/1952 Robbins 338/195 3,394,386 7/1968 Weller 317/261 X 3,411,947 11/1968 Block 338/308 X 3,456,170 7/1969 Hatch 317/261 X 3,509,511 4/1970 Soroka 338/195 X OTHER PUBLICATIONS IBM Tech. Disclosure Bulletin, Vol. 24, No. 9, Feb.

1962, pp. 15 & 16.

Primary ExaminerE. A. Goldberg Attorney, Agent, or FirmJoseph S. Iandiorio 57' ABSTRACT A method of manufacturing a precision resistor element of a predetermined resistance value, the resistor element including a quantity of resistive material extending between two terminals comprising removing a first portion of the resistive material in a first area between the terminals for substantially increasing the re sistance to obtain a first resistance value for the resistor element, and removing a second portion of the resistive material in a second area alongside of the first area and between the first area and one of the terminals for slightly increasing the resistance to obtain the predetermined resistance value.

1 Claim, 9 Drawing Figures PAIENIED I974 3. 787, 965

sum 2 0F 2 IOV Theodore Cocoa Pou/ J. Sanders uvvmrms 4 ana z'arz'a ATTORNEY.

METHOD OF MAKING RESISTOR FIELD OF INVENTION This invention relates to a precision resistor element and a method of manufacturing it, and more particularly to a more precise technique for trimming resistor elements.

BACKGROUND OF INVENTION Typically, thick film resistor elements are manufactured by screening a resistor material onto a substrate, baking and drying the material, and then firing it at a high temperature followed by the screening of a glass coating over the resistor material, baking and drying the coating and then firing it at a lower temperature. After a resistance element has been completed, its resistance is measured. If the resistance is too low, a bit of the resistive material is abraded away until the desired resistance value is reached. Then the exposed abraded area is glass coated again and the resistor element is fired again. Even though the resistor materials have a very uniform consistency and precisely defined resistance per unit volume, the screening takes place in a controlled environment, and the amount and distribution of the resistor material deposited is closely controlled, the manufacturing tolerances are still sometimes insufficient. The trimming operation is usually performed by an abrasive device that wears away a part of the resistor material with ajet of sand or other abra sive material discharged under pressure through a nozzle but many other methods such as laser trimming, cutting, and chemical etching may also be used to remove resistor material. Generally the nozzle and resistor material are moved relative to one another to cut or wear away a portion of the resistor material in a path transverse to the intended current direction through the material. As the movement continues the path becomes longer and the cross sectional area of the resistor material is decreased until the desired predetermined resistance value of the resistor element is reached. In order to promote greater precision all aspects of the process have been improved as the technology progressed including the use of: better, more uniform resistor materials, better screening techniques, finer abrasive materials, more sharply focused nozzles, low inertia nozzle carriages, better reference resistors and better feedback control circuits to drive the nozzle. But still greater precision is required.

SUMMARY OF INVENTION It is therefore an object of this invention to provide an improved precision resistor and a method of making It.

It is a further object of this invention to provide an improved method of trimming a resistor to the desired predetermined resistance value.

It is a further object of this invention to provide such an improved trimming method which is simple, precise, inexpensive, and requires no new equipment.

It is a further object of this invention to provide such an improved trimming method for obtaining a much finer adjustment of the final resistor resistance value and a much more precise resistor.

It is a further object of this invention to provide such an improved resistor and method of making it which results in a more precise and more stable resistor.

This invention features a precision resistor element and a method of manufacturing such a resistor element of a predetermined resistance value. The resistor element includes a quantity of resistor material extending between two terminals. A first portion of the resistor material is removed in a first area between the terminals for substantially increasing the resistance of the resistor element to obtain a first resistance value for the resistor element. A second portion of the resistor material is removed in a second area alongside of the first area and between the first area and one of the terminals for slightly increasing the resistance of the resistor element to obtain the predetermined resistance value.

DISCLOSURE OF PREFERRED EMBODIMENT Other objects, features and advantages will occur from the following description of a preferred embodiment and the accompanying drawings, in which:

FIG. 1 is a plan view ofa typical substrate containing a plurality of resistor elements.

FIG. 2 is an enlarged view of a portion of a substrate with two spaced conductors as it appears before the resistor material is applied.

FIG. 3 shows the substrate portion of FIG. 2 after the resistor material has been screened onto it.

FIG. 4 is a view of the substrate portion of FIG. 3 after the resistor material has been baked and dried and a glass coating has been screened onto it.

FIG. 5 is an enlarged cross sectional view taken along line 5-5 of FIG. 4.

FIG. 6 is a view of the substrate portion of FIG. 4 after the glass coating has been baked and dried, the resistor'material and glass coating have been co-fired and a portion of the resistor material and surrounding glass coating have been removed in the first step of the trimming operation according to this invention.

FIG. 6A is a view of the substrate portion of FIG. 6 after the area removed has been sealed with a glass coating.

FIG. 7 is a view of the substrate portion of FIG. 6 after subsequent steps of the trimming operation according to this invention.

FIG. 8 is a view of the substrate portion of FIG. 7 after the area removed in the trimming operation has been resealed with a glass coating.

The trimming technique of this invention is described herein with reference to thick film circuits, but this is not a limitation of the invention; it may be used for thin film circuitry, hybrid circuitry, discrete elements and many other applications. There is shown in FIG. 1 a substrate 10 containing a plurality of resistor elements 12 according to this invention. Each resistor element 12 includes a resistor material or paste 14 applied between a pair of conductors 16, 18 and a high temperature dielectric glass coating 20 covering the resistor paste l4. Resistor paste 14 may be DuPont Birox paste or another Ruthenium oxide paste.

The trimming technique of this invention may be best illustrated by step-by-step explanation of the construction of a single resistor element. Initially, a pair of conductors 16, 18 are established on a substrate 10, only a portion of which is shown in FIG. 2. Following this, resistor plate 14 is screened through a mask to fill the space between conductors l6 and 18, FIG. 3. Next, the conductors 16, 18 and resistor paste 14 supported on substrate 10 as shown in FIG. 3 are baked and dried, typically, at approximately C. for approximately 30 minutes. After this baking a high temperature low dielectric glass 20, FIG. 4, is screened over the resistor paste 14 using a mask similar to the one used to apply the resistor paste. The substrate with conductors 16, 18, resistor paste l4 and glass coating is now baked and dried at approximately 125 C. for approximately 30 minutes. Following this baking, the entire substrate 10 as pictured in FIG. 4 is fired at 840 C. for approximately 10 minutes so that the glass coating 20 and the resistor paste 14 are both simultaneously co-fired. This co-firing process causes a merging of the glass coating 20 and the resistor paste 14 at their interface 22, FIG. 5, instead of the sharp boundary 24, shown in phantom, which is obtained when the resistor paste l4 and the glass coating 20 are separately fired. This merging at interface 22 contributes to a more uniform gradient of the coefficient of expansion between resistor paste 14 and glass coating 20 which substantially reduces stresses between coating 20 and paste 14. The elimination of such stresses results in the reduction of strains in the paste 14 which can vary the resistor characteristics of paste 14. Another advantage of this process is that it results in a better, more predictable temperature coefficient of resistance of the resistor element.

After the final firing is completed the element 12 is submitted to a testing device which compares its resistance to that of some reference resistance. If the resistance of element 12 is lower than the reference resistance, a portion of paste 14 is removed in an area generally transverse to the extent of paste 14 between terminals l6 and 18 leaving a notch 26, FIG. 6. The removal of paste 14 in notch 26 directly reduces the effective cross-sectional area of paste 14 presented to the current flow as well as eliminates a finite quantity of the paste 14. As a result of both of these conditions the resistance of element 12 is substantially increased toward that of the reference resistance with a fair amount of precision. In order to reach that reference resistance value with even greater precision a second portion of paste 14 may be removed in a second area alongside notch 26 leaving a notch 28, FIG. 7. The creation of this second notch 28 in the shadow of notch 26 with respect to the current flow through paste 14, also reduces the resistance of element 12 but to a much lower degree. The lesser effect of the second removal of paste 14 in notch 28 results from the fact that it is in the shadow of notch 26 and does not further reduce the cross-sectional area presented to the current flow. However, there is still associated with the creation of notch 28 the removal of the finite quantity of paste 14 constituting resistor element 12 and thus there is some increase in resistance. However, the increase of resis tance of element 12 per quantity of paste removed in creating notch 28 is much less than for equal quantities removed in creating the first notch 26: the trimming operation conducted in the second area containing notch 28 is a much finer adjustment; the increase in resistance proceeds at a much slower rate than in the creation of the first notch 26.

As the length of the second area, notch 26, approaches that of the first, notch 28, the rate of increase of resistance increases; if the second area becomes coextensive with the first its finer trimming function abates.

If even greater precision is required any number of subsequent areas may be used. For example, a third area creating notch 30 may be used alongside of notch 28. In this manner a number of notches may be created to improve the precision of a resistor element. The notches need not be so close alongside each other that they merge into each other as is the case with notches 26, 28 and 30. They may be alongside of each other but spaced apart; second notch 28 may be created as notch 28',-FIG. 7. All of the paths should be in each others shadow, alongside each other: a subsequent notch cut on the opposite edge of paste 14 of element 12 would more substantially increase the resistance than if it were on the same edge as the existing notches.

The removal of the paste 14 may be accomplished by any suitable means known to those skilled in the art including chemical techniques such as etching, mechanical techniques such as cutting or milling, abrasive techniques such as now widely used in the thick film resistor industry. Although the areas and notches have been referred to as if they were cut from the edge of paste 14 inwardly this is not a limitation of the invention. For example, in chemical or mechanical processes the paste 14 may be etched or scraped away, respectively, from the top down rather than inwardly from an edge. Indeed, it is not even requried that an edge be involved at all, as the removal may be accomplished wholly within the area of the paste 14 so that instead ofa notch with paste 14 on three sides there may result a hole with paste 14 on all sides, and the notches need not all be of the same width. Further, the size or shape of the recess remaining after the removal is not restricted to a rectangular hole or notch: the result is the same whether or not the resulting recesses are regular geometric shapes or wholly irregular, and whether or not all recesses are alike, or are created in the same way or in the same direction. For example, after the first notch 26 is created by moving an abrasive discharge nozzle inwardly from the edge of paste 14, in the direction of arrow 32, the second notch 28 can be created by retracting the nozzle a bit and then moving it in the direction of arrow 34 transverse to that of arrow 32. It is not the direction of the removal action that is relevant but rather it is the position of the second and subsequent removal sites relative to the previous removal site that is important.

Notches 26, 28 and 30 leave exposed a portion of resistor paste 14 which can now be sealed over with a lower temperature glass which sets at a temperature considerably lower than 840 C., typically 525 C. One such glass is DuPont 8185. After coating notches 26, 28 and 30 with this low temperature glass 36, FIG. 8, substrate 10 can be fired to 525 C. for approximately 10 minutes to set the glass coating 36 without interfering with resistor paste l4 and glass coating 20 which was set at 840 C., thereby eliminating the danger of disturbing the previously established resistance value of resistor paste 14.

The stability of the resistance value may be improved by applying the second or low temperature glass coating to the resistor after the first portion of paste is removed but prior to the removal of second and subsequent portions of the paste. It is the first and deepest cut or notch which removes the most paste and which defines the greatest part of the resistance value of the resistor; other additional notches or removals serve to vary the resistance value only to a very small degree. Thus by coating the resistor after the first, most significant cut is made the exposed border of this most vulnerable area is protected and the greater part of the re- Other embodiments will occur to those skilled in the art and are within the following claims:

What is claimed is:

1. A method of manufacturing a precision resistor element of a predetermined resistance value comprising:

applying resistor material between two terminals on a substrate;

baking and drying the resistor material;

applying a glass coating on the resistor material;

baking and drying the glass coating;

co-firing the material and coating a first temperature to form a resistor element;

removing a first portion of said resistor material between said terminals in a path transverse to the extent of said resistor material between said terminals for increasing the resistance of said element to at least a first resistance value less than said predetermined resistance value and removing a second portion of said resistor material in a second path shorter than and adjacent to said first path, between said first path and one of said terminals for increasing the resistance to said predetermined resistance value; and

applying a low temperature glass coating to cover said resistor material exposed by the removal of said portions, baking and drying the low temperature glass coating at approximately C., and firing the resistor element and low temperature glass coating at a temperature of approximately 525 C.

which is lower than said first temperature. 

1. A method of manufacturing a precision resistor element of a predetermined resistance value comprising: applying resisTor material between two terminals on a substrate; baking and drying the resistor material; applying a glass coating on the resistor material; baking and drying the glass coating; co-firing the material and coating a first temperature to form a resistor element; removing a first portion of said resistor material between said terminals in a path transverse to the extent of said resistor material between said terminals for increasing the resistance of said element to at least a first resistance value less than said predetermined resistance value and removing a second portion of said resistor material in a second path shorter than and adjacent to said first path, between said first path and one of said terminals for increasing the resistance to said predetermined resistance value; and applying a low temperature glass coating to cover said resistor material exposed by the removal of said portions, baking and drying the low temperature glass coating at approximately 125* C., and firing the resistor element and low temperature glass coating at a temperature of approximately 525* C. which is lower than said first temperature. 